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"The world will change you -- you have to adapt or die," the actress tells THR in our weekly postmortem.

[Warning: This story contains spoilers from episode 414, "The Grove," of AMC's The Walking Dead as well as the comics it is based on.]

AMC's The Walking Dead delivered one if its most gut-wrenching episodes of its fourth season Sunday when Carol (Melissa McBride) and Tyreese (Chad L. Coleman) thought they'd found a new place to settle down and live a very humble life with Lizzie, her sister, Mika, and Rick's daughter, Judith.

But as fans of the zombie drama have come to learn, no one is ever really safe in this new postapocalyptic world. A new threat emerged as Lizzie (Brighton Sharbino) and, to a certain extent, Mika (Kyla Kenedy) illustrated that they could not adapt to the new rules of the world and do what it takes to survive.

During the hour, Carol and Tyreese, exhausted from trying to survive on the road with three young girls, stumble on a cabin that's fully stocked, with gas and running water, and seek salvation there while debating the merits of Terminus. What they find, however, is that Lizzie was the one feeding rats to the walkers at the prison and the youngster still doesn't understand that walkers are deadly, and neither of them want to kill anyone, including the undead.

After Carol and Tyreese briefly leave the girls, they come back to find that Lizzie has killed her kid sister, Mika, but insists that she'll "come back" because she "didn't hurt her brain." Lizzie, with her hands covered in blood, holds Carol and Tyreese at gunpoint with baby Judith nearby and insists that they wait to see Mika return in order to truly understand that the walkers are not dangerous. Worse: Lizzie reveals that Judith can change, too, and she was just about to kill her as well.

It's at that moment that Carol and Tyreese realize that Lizzie is incapable of taking care of herself in this world and is now a threat to everyone else. After briefly debating alternatives, they come to the same conclusion: Lizzie must be killed. When Carol takes Lizzie for a walk, Lizzie apologizes -- not for killing her sister but for holding them at gunpoint. With tears streaming down Carol's face, she shoots Lizzie in the back of the head.

Carol and Tyreese realize that they can't stay at the cabin, and Carol then confesses to Tyreese that she was the one -- not Lizzie as he'd speculated -- who killed his girlfriend, Karen. Carol hands Tyreese her gun and leaves him to decide if she should live or die. He not only decides that she should live, but he also forgives her for trying to protect the group from the deadly virus. The two, with Judith, head for Terminus.

The Hollywood Reporter caught up with McBride to discuss Carol's devastating decision, how this will change her moving forward and what she and Tyreese will expect from Terminus.

How did you react when you first heard about what Carol had to do in this episode?

Overwhelmed and devastated. I sat there reading the script with my mouth open. Shortly after Rick (Andrew Lincoln) banished Carol from the prison, [showrunner] Scott [M. Gimple] told me Carol was going to come back and something pretty bad was going to be required of Carol.

How will Carol having to execute Lizzie change her going forward? Can she come back from that?

It was something that had to be done in that world and under those circumstances; Lizzie in that world seemed inevitable. It would be impossible for Tyreese and Carol to move forward with Judith, who doesn't have any experience of the world before the apocalypse. It was so devastating for Carol to have to do that. Moving forward, Tyreese forgave Carol after she confessed to killing Karen. She put her fate in his hands and gave him the choice of, "Do you think that I'm worthy of living?" He forgave her and he understood. That brings the whole humanity back. Moving forward, it's a new beginning.

Did Carol have any option other than killing Lizzie?

No, I don't think there was really any other option. There's a lot of nature versus nurture going on in this episode to look at. As much as it broke Carol's heart to have to do this and to realize this had to be done. They were walking toward the flowers in that scene and Lizzie says, "You're mad at me and I'm sorry." You'd think she'd be sorry for stabbing her sister to death, but instead she's sorry for pointing gun at her, so she just doesn't get it.

Why do you think Lizzie and Mika could never wrap their heads around the new rules of the world?

They're two very different people. You have one child who is afraid to hurt anything and another who doesn't mind killing people as long as they come back as a walker and live forever. It made me wonder what happened to their mother because she played an important role in that episode as well for me. When Mika first says, "Everything works out the way it's supposed to," that was the last thing Carol said to Lizzie. There was something mystical about that episode to me with the smoke burning and later when we're walking toward the grove and it's out. The house we found in the grove being so idyllic and having every sustainable thing we needed there was a little too good to be true. The idea that we can stay there, and that brief happiness you saw was also too good to be true. That line, "Everything works out the way it's supposed to" -- for that to be on Carol's mind at that time was almost like a mom-to-mom kind of conversation.

As a mother who lost her daughter (Sophia) and then had to watch her be put down, what kind of emotional damage might the Lizzie and Mika situation have on Carol now? Will this reopen old wounds?

In a very big way, especially with that line from their mother, it almost helps Carol to close that old wound in a way. "Everything works out the way it's supposed to" -- in the context of all that went down and thinking it was inevitable that these children would lose their lives because they couldn't do what it took to defend themselves. Mika said she couldn't and would never kill another person, and she couldn't defend herself from her sister, either. It just seemed like it was something that would be inevitable.

Carol blamed herself for not seeing how damaged Lizzie and Mika were. Will this close her off even more when she meets new people, especially children? How can she recover from this?

I'm really interested to see how her mindset is moving forward after this. You have Judith, and it's like trying to figure out again how we adapt to this world. You have children now who are coming up who aren't aware of the world before the apocalypse. They'll experience these horrible things, and how does that redefine trauma when it's something that's commonplace? For all I know, Lizzie could have been traumatized by her mother's death. Maybe that was the last thing her mother said to her -- what is this thing about bringing the walkers back? This already hard-wired nature [that Lizzie had] to destroy animals and that sort of thing -- that postapocalyptic world would nurture that child in the worst way -- specifically a child like her. Then you have children who are born into the apocalypse. It's going to be interesting. The children are the hope of the future, and to protect them is so important to Carol.

Carol did everything she could to help the girls, but nothing could save them. What's the message here, that this world is not fit for children?

It's not a world that's safe for anyone. The ability to fight isn't a one-size fits all; everybody is different. Thematically, there's a lot said about change. Something I got out of this episode for Carol, too, is that you have to change. The world will change you -- you have to adapt or die. It's about hanging on to that part of yourself: You can change but don't lose yourself. That's what was happening to Carol -- her mindset -- she was so hell-bent on protecting these children that she lost a bit of something, and that was her nurturing aspect. She was missing a lot of stuff because her eyes were so set on survival.

Carol finally told Tyreese that she killed Karen. She gave Tyreese the option to kill her, handing him her gun. Do you think Carol wanted -- and deserved -- to die?

It was about letting him determine her fate; that was the only way to move forward. Carol by then knew it had to be done at that moment. Carol felt like if Tyreese is the person that she has come to know -- he has seen what they've just been through -- then he's not going to kill her. But she leaves that to him. By giving him the choice, it puts Tyreese in her position in a way. By then she feels like he understands, and she's using that mindset to give him the choice: Do what you feel you have to do. But what she wasn't expecting was forgiveness.

Lizzie was revealed to have been the one feeding rats to walkers at the prison, which helped weaken those walls. Will this be something that gets back to Rick when and if the group is reunited?

I would imagine if they're reunited and he's asking, she would tell him everything that happened.

As a father, how might Rick respond to learning of what Carol had to do? Could that make the wedge between them bigger?

We'll have to wait and see. There's a lot going on between the time he banished Carol. Even when he comes back to the prison, he turns to Maggie and says, "There's no time to doubt yourself," which I thought was great after Carol gives him the watch with how she sees time. There was no time for her to doubt herself when she put Karen and David down. A lot has happened. Maybe perspectives are changing. We'll see.

Carol and Tyreese are now on the road to Terminus. Given their experience here, how much would you say they trust each other now?

They trust each other 100 percent now. That was one of the reasons why it was so important for Carol to confess. She knew that she had to be able to trust the people she's with. He didn't kill her. Carol and Tyreese have bonded incredibly over this tragic experience and, as far as they know, the only two people. The only way to move forward is to be able to trust whomever you're with 100 percent.

They thought the cabin could be a good sanctuary, but that turned out to be too good to be true. Might they approach that a bit more cautiously?

I totally believe that. Tyreese has every reason to as well. He was taken in by Woodbury, and that was insane.

Where was baby Judith?

Tyreese is carrying her on his back, but she's with them.

Daryl (Norman Reedus) is still out there and facing a threat in a new group of thugs. How might Carol react when and if she learns that nobody knows where he is?

She's worried about everybody in her group. There's a part of her that believes he can handle himself alone, but the threats in this world are so many and they're so horrifying. It's anybody's guess, and there's always heartbreak with the hope in a way.

What might Carol and Daryl's reunion look like, when and if that does come to pass?

Bittersweet, maybe. He's probably very curious about what happened with Rick and Carol and the banishment and all that stuff. But I think that they'd be very happy to see that they're still alive.

What did you think of Carol's decision? Do you think she had a choice? Click here to vote in THR's poll and hit the comments section below with your thoughts. The Walking Dead airs Sundays at 9 p.m. on AMC.